Cardioversion generally refers to the discharge of relatively high energy electrical shocks into or across cardiac tissue to arrest a tachyarrhythmia of a cardiac chamber. The termination of high rate tachycardias with lesser energy electrical pulses or bursts has also been referred to as cardioversion. The arrest of atrial or ventricular fibrillation by higher energy shocks is referred to as defibrillation, and defibrillation has been characterized in the past as a form of cardioversion. Implantable cardioverter/defibrillator (ICD) systems are available for providing synchronous cardioversion shocks and/or asynchronous defibrillation shocks. Additionally, pacemaker/cardioverter/defibrillator (PCD) systems are available for providing additional staged therapies of anti-tachyarrhythmia pacing, synchronous cardioversion shocks and asynchronous defibrillation shocks.
ICD's or PCD's will typically include a housing assembly containing a microprocessor or control unit and a power supply. One or more leads are coupled to the housing assembly and positioned within or adjacent to a portion of the heart. The leads may include electrodes to deliver directed discharge of electrical current. The housing assembly itself can be formed from a conductive material and may serve as an electrode. When so utilized, the housing assembly is often referred to as an “active can” or can electrode. For example, an electrode may be positioned in the right ventricle (RV). A high voltage shock may then be directed from the can electrode to the RV electrode across a major portion of cardiac tissue. Of course, electrodes can be placed in any chamber of the heart and the defibrillating or cardioverting shock can be managed and administered as appropriate between various electrodes.
The variety of lead/electrode configurations available on ICD's or PCD allow for a wide range of treatment options directed specifically to a patient's particular cardiac condition. This variety also requires that certain common assemblies be implemented for use in varying contexts and programmabilities to accommodate specific configurations. In addition, it is desirable to determine the operative and functional status of the various components.
Certain implantable medical devices have previously performed some self diagnostic procedures. For example, commonly assigned U.S. Pat. No. 5,755,742 discloses a system that indirectly measures the impedance of certain leads coupled to an implantable pacemaker/cardioverter/defibrillator. A low voltage or sub-threshold pulse is generated across a given pair of leads. Another pair of leads then monitors that pulse and derives the impedance from the measured values. One lead is common to both the generating lead pair and the measuring lead pair. The derived impedances are used to assess and evaluate the integrity of the lead being tested.